Current electronic circuits often need more than one direct-current (DC) power supply, and therefore various systems are designed for power conversion, for example charge pumps. A conventional charge pump is switched between a charging phase and a discharging phase so as to operate in two phases, i.e., the charging phase and the discharging phase by turns. For example, in a 1.5x mode charge pump as shown in FIG. 1, two capacitors C1 and C2 have a same capacitance C, and the difference between the maximum voltage and the minimum voltage on the capacitors C1 and C2 is ΔV1. This charge pump operates in a charging phase when switches 10, 16 and 22 turn on and the others turn off, in which a charging path 11 is established to charge the serially coupled capacitors C1 and C2. Let the charging time to be T1, therefore the charging current Iin1 is C×ΔV1/T1. Contrarily, the charge pump operates in a discharging phase when the switches 12, 14, 18 and 20 turn on and the others turn off, in which a discharging path 21 is established to discharge the parallel coupled capacitors C1 and C2. Let the discharging time to be T2, therefore the discharging current Iin2 is 2×C×ΔV1/T2. The input current Iin is equal to (Iin1×T1+Iin2×T2)/(T1+T2), and the output current lout will be 2×C×ΔV1/(T1+T2). FIG. 2 shows a relationship between the input current Iin and time. If T1=T2, the charging current Iin1 will be equal to the output current lout, and the discharging current Iin2 is two times of the output current lout. In the charging phase T1, Iin=Iin1=Iout; while in the discharging phase T2, Iin=Iin2=2Iout. Hence, this charge pump has huge current ripple. Since the voltage ripple is proportional to the current ripple, the charge pump also has huge voltage ripple, which causes high noise. To improve this problem, U.S. Pat. No. 6,504,422 to Rader et al. proposes a charge pump which is switched by a switching circuit to charge a capacitor and discharge another in two phases, in order to reduce the voltage ripple. However, it still cannot improve the current ripple.
Therefore, it is desired a charge pump and a control method thereof which can improve the current ripple and the voltage ripple to reduce noise.